Magnetic disk recording systems for storing digital data incorporate magnetic recording and playback heads which must be accurately positioned over tracks on the disk whereon data has been recorded or is to be recorded. These heads must be rapidly moved (generally along a radial line) from one position to another, responsive to control signals, so that data can be read from and written at appropriate locations on a disk. Typically, linear electromechanical positioning mechanisms are employed for effecting the required head motion. A closed loop servo system is used to control the positioning mechanism; and this servo frequently requires a velocity sensing transducer, for detecting, controlling and stabilizing head motion.
Typical prior art velocity tranducers or rate of motion detectors employ a moving magnet which induces a voltage in a long, stationary coil. The induced voltage is proportional to the velocity of the moving magnet and this, in turn, is directly related to the velocity of the head. These transducers, however are relatively long--more than twice as long as their rated sensing distance. In addition, their sensitivity to external noise fields is frequently objectionably high.
A general, diagrammatic illustration of a typical magnetic disk recording system (in which the present invention would see application) is provided in FIG. 1. As illustrated therein, a magnetic disk recording medium 12 is rotated at a high rate of speed by a drive motor 14 on a spindle 16 turned by the drive motor. A magnetic recording head 18 is used to record (i.e., write) signals on and read signals from the disk 12. Head 18 typically floats upon a cushion of air set up by the rapid rotation of the disk. A servo-mechanism 22, responsive to position signals provided on line 24, controls the motion of head 18 through an electromechanical 26 including a head positioning link 28. By action on link 28, the servo-mechanism reciprocates the head 18 in the radial direction indicated in the figure by the letter X and a double-ended arrow 29, in order to position the head at a desired one of many concentric tracks on the disk.
A velocity sensor or transducer 30 is also connected to linkage 28. This sensor generates an electrical output signal responsive to the motion of linkage 28 and, consequently, head 18. The output signal from velocity transducer 30 is fed back to the servo control 22; the servo computes and generates any required correction signals to drive the head 18 toward the commanded position, if it is to counteract any motion or deviation away therefrom.
FIG. 2 shows one basic alternative arrangement for a prior art velocity transducer which, by contrast to the moving magnet rate detector, employs a moving coil 32 and fixed magnet 34. The magnet 34 is attached between a pair of iron pole pieces 36 and 38 which extend longitudinally, in parallel, in the X (i.e., radial) direction. The magnetic flux between the north pole N and the south pole S of the magnet 34 is concentrated by the pole pieces 36 and 38 across the air gap 40 which exists between the pole pieces. The arrows 42 between the pole pieces 36 and 38 indicate the magnetic field in the region 40. The coil 32 is mechanically connected to the linkage 28 by conventional means, not shown in order to preserve the clarity of the illustration. Thus, the linkage 26 moves the coil 32 along the pole piece 36 in the X direction. As a result of the motion of the coil 32, a voltage E appears across its terminals 44 and 46.
The induced voltage E is given by the following relationship: EQU E (d.PHI./dt)=(d.PHI./dx).multidot.(dx/dt)=(d.PHI./dx).multidot.v
where .PHI. represents the magnetic flux and v represents linear velocity in the X direction. Thus, for a constant magnetic flux density B in the air gap 40, a constant change in flux would be realized and the transducer would produce a signal proportional to the velocity.
While the sensor of FIG. 2 is shorter in length and less expensive to fabricate then a moving magnet transducer with a stationary coil, it nevertheless has significant drawbacks. First, the magnetic flux density B cannot be made sufficiently constant in the air gap to achieve the head positioning accuracy and stability often desired in current disk system technology. Second, the iron pole pieces saturate near the section 48 and reduce the amount of flux that bridges the air gap, also thereby reducing the sensitivity of the detector. Third, the detector is quite sensitive to radiation from external alternating electromagnetic sources such as the voice coil (i.e., positioner) which is commonly employed in the electromechanical actuator 26 to drive the linkage 28.
An improvement in sensitivity is accomplished with the design shown in FIG. 3, by the use of a different magnetic flux path. In that structure, magnet 52 produces flux spanning the total range of travel of the moving coil 54 and pole pieces 56 and 58 provide a closed path for returning the flux around the ends of the magnet. Assuming that the iron path pieces 56 and 58 are of like cross-sectional area and configuration as pole pieces 36 and 38, this tachometer has typically twice the output per unit velocity as compared with the design shown in FIG. 2. Further, as the iron pieces 56 and 58 are driven into (plus and minus) saturation in the end regions 62 snd 64, respectively, a reduction in noise sensitivity is also achieved.